I suppose that Rydberg atoms are the best way to find by experiments the largest distance of influence of the electric field of a single electron in a electric dipole. Furthermore how the electrical positive charged nucleus influence this distance? Are there experiments with helium molecules or atoms with only one Rydberg electron?
Update: According to the below fig. from Wikipedia, how there can be a finite ionisation level if the electric field of an electron is infinite? It has to be an empiric value?